A brushless direct current (DC) motor control system typically includes a DC source that supplies power to an inverter. The inverter converts the DC power to alternating current (AC) power having corresponding bi-directional current waveforms that are provided respectively to coils of a brushless DC motor. The inverter includes switches that are controlled by a controller. The controller generates pulse width modulation (PWM) control signals, which are provided to each of the switches. The controller adjusts duty cycles of the PWM signals to adjust the speed of the motor.
Sensors, such as Hall effect sensors, are suitably used to determine the position and speed of the brushless DC motor. The controller, for example, typically generates the PWM signals based on the determined speed and/or a predetermined back electromotive force (BEMF) profile of the brushless DC motor. BEMF of a brushless DC motor refers to an amount of resistance to rotation of a rotor of the motor relative to the speed of the motor. As speed of the motor increases, resistance to the rotation of the rotor increases. A BEMF profile is a plot relating BEMF of a motor to speed of the motor. As the speed of a motor (or a duty cycle of the PWM signals of the motor) increases, the amount of BEMF increases. Examples of BEMF profiles of different motors are shown in FIG. 5.
To maximize operating efficiency of a brushless DC motor (hereinafter referred to as a “motor”), a driving profile of the motor is generated to match a BEMF profile of the motor. The driving profile refers to duty cycles of one or more control signals provided to switches of an inverter, which is used to drive the motor. Different types of motors have different BEMF profiles. In addition, different motors of the same type have different BEMF profiles due to, for example, differences associated with manufacturing tolerances and/or differences in manufacturing at different facilities.
A large amount of storage is needed to store the BEMFs for the different motors. For this reason and to minimize the amount of storage needed, a manufacturer selects a single predetermined BEMF profile and applies that same BEMF profile to different motors. Control systems manufactured by the manufacturer of the different motors store the single predetermined BEMF profile. The single predetermined BEMF profile typically does not match the BEMFs of the motors, but rather is a BEMF profile used to approximate the BEMFs of the motors. Since each motor is driven based on a BEMF profile that does not match an actual BEMF of that motor, driving efficiency is not maximized. Driving efficiency of a motor refers to an amount of power out of the motor relative to an amount of power in to the motor.